Copolymers of vinylidene fluoride and hexafluoropropylene and process for preparing the same

ABSTRACT

Melt processable copolymers of vinylidene fluoride and hexafluoropropylene, containing from about 1% to about 20% hexafluoropropylene by weight, are prepared by emulsion polymerization of vinylidene fluoride and hexafluoropropylene in a stirred aqueous reaction medium. The reaction medium is formed by charging to a heated reactor, water, vinylidene fluoride, an initiator to start the polymerization, and a water-soluble surfactant capable of emulsifying both the initiator and the reaction mass during polymerization. Additional amounts of vinylidene fluoride and initiator are fed to the reaction medium to continue polymerization of the vinylidene fluoride monomer until from about 50% to about 90% of the total weight of the vinylidene fluoride monomer utilized in the process has been added to the reaction medium. There is then added to the reaction medium (i) from about 1% to about 20% hexafluoropropylene by weight, based upon the combined weight of the hexafluoropropylene and the total weight of the vinylidene fluoride utilized in the process, and (ii) the balance of the vinylidene fluoride utilized in the process. The resulting vinylidene fluoride/hexafluoropropylene copolymers are characterized by improved mechanical properties and a DSC melting point typically in the range of from about 160° C. to about 170° C., preferably from about 163° C. to about 168° C.

This is a continuation of application Ser. No. 08/065,700, filed May 21,1993, now abandoned which is a continuation of application Ser. No.07/799,452, filed Nov. 26, 1991, now abandoned, which is a divisional ofapplication of application Ser. No. 07/521,814, filed May 10, 1990, nowU.S. Pat. No. 5,093,427.

FIELD OF THE INVENTION

The invention relates to the preparation of vinylidene fluoridepolymers, more particularly to the preparation of copolymers ofvinylidene fluoride and hexafluoropropylene having improved physicalproperties.

Abbreviations:

The following materials are referred to in the herein specification bytheir common abbreviations:

DSC differential scanning calorimetry DTBP di(tert-butyl)peroxide HFPhexafluoropropylene IPP diisopropyl peroxydicarbonate TCFMtrichlorofluoromethane VDF vinylidene fluoride PVDF polyvinylidenefluoride

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 4,076,929 and 4,569,978 describe an improved emulsionpolymerization process for preparing VDF homopolymers and copolymershaving improved flexibility. According to the aforesaid process, acomonomer for VDF is added in an emulsion polymerization mixture as partof an initial charge, or the comonomer is introduced continuously in afixed ratio with respect to VDF. The comonomer may comprise HFP. Theinvention of U.S. Pat. No. 4,076,929 results in extrusion grade polymerresins having improved melt flow characteristics which may be utilizedfor the fabrication of objects having greater flexibility and impactresistance. These improvements are diminished as the melt viscosity ofthe polymer is decreased to 20 kilopoise or lower, measured at 232° C.and 100 sec⁻¹ (ASTM 3835), unless increasing amounts of HFP comonomerare utilized. However, as the amount of HFP in the polymer is increased,the DSC melting point, and thus the use temperature of the polymer,decreases.

What is needed is a VDF/HFP copolymer having improved physicalproperties, particularly improved melt flow, which improved propertiesare obtained without significant reduction in polymer use temperature,as compared to the use temperature of VDF homopolymer. Moreparticularly, there is a need for a VDF-based polymer which has improvedmelt flow, but which substantially maintains the melting point of VDFhomopolymer.

SUMMARY OF THE INVENTION

A process is provided for the production of VDF/HFP copolymer by theemulsion polymerization of VDF and HFP in a stirred aqueous reactionmedium. The aqueous reaction medium is formed by charging the followingto a reactor: water, VDF, an initiator to start the polymerization, anda water-soluble surfactant capable of emulsifying both the initiator andthe reaction mass during polymerization. Additional amounts of VDF andinitiator are fed to the reaction medium to continue polymerization ofthe VDF until from about 50% to about 90% of the total weight of the VDFutilized in the emulsion polymerization process has been added to thereaction medium. There is then added to the reaction medium (i) fromabout 1% to about 20% HFP by weight, based upon the combined weight ofthe HFP and the total weight of VDF added to the reaction medium in theemulsion polymerization process, and (ii) the balance of the VDFutilized in the process. VDF/HFP copolymer is then obtained from thereaction medium. The HFP is preferably added to the reaction medium asrapidly as possible. The balance of the VDF monomer is preferably addedto the reaction medium at the same or substantially the same rate asadded during the VDF homopolymerization phase of the process.

Preferably, from about 65% to about 85%, most preferably from about 70%to about 80%, of the total weight of VDF utilized in the process isadded to the reaction medium before the addition of any HFP. From about5% to about 15% HFP by weight is preferably added to the reactionmedium, based upon the combined weight of the HFP and the total weightof VDF added to the reaction mixture during the polymerization process.The aqueous reaction medium for polymerizing the VDF monomer preferablycontains a chain transfer agent, e.g. TCFM, for controlling themolecular weight of the resulting polymer.

According to one embodiment of the invention, VDF is added to thereaction mixture, prior to the addition of HFP, as an initial VDF chargesufficient to obtain an operating pressure in the reactor of at leastabout 450 PSIG, preferably, from about 450 to about 700 PSIG, afterwhich initiator is added in an amount equal to from about 0.25 to about2.0 grams per kilogram of VDF present in the initial charge. VDF isadditionally added to the reaction medium as a continuous or incrementalVDF charge to increase the amount of VDF added to the reaction medium tofrom about 50% to about 90% of the total amount of VDF utilized in thepolymerization process. Additional initiator is added to the reactionmedium to continue polymerization of the VDF.

Establishment of the reaction medium prior to addition of the initialVDF charge may comprise the steps of initially charging the reactor witha reaction medium comprising water, fluorosurfactant, and paraffin wax;agitating and heating the mixture; ceasing agitation and venting thereaction medium; resuming agitation and adjusting the temperature to anoperating polymerization temperature of from about 65° C. to about 150°C.; and optionally adding chain transfer agent in an amount sufficientto obtain the desired molecular weight of polymer.

The invention further relates to VDF/HFP copolymers such as thoseprepared according to the present invention, having a HFP content offrom about 1% to about 20% by weight, preferably from about 1% to about15%, most preferably from about 5% to about 15%, and a melting point inthe range of from about 160° C. to about 170° C., preferably from about163° C. to about 168° C.

By “VDF/HFP copolymer” or “vinylidene fluoridehexafluoropropylenecopolymer” is meant normally solid polymers containing at least 50 molepercent vinylidene fluoride copolymerized with hexafluoropropylene as acomonomer, and optionally containing one or more further comonomersselected from the group consisting of tetrafluoroethylene,trifluoroethylene, chlorotrifluoroethylene, vinylfluoride,pentafluoropropene, and any other monomer that will readily copolymerizewith vinylidene fluoride. Most preferably, said further comonomer(s)is/are selected from those monomers which are at least as reactivetoward polymerization as vinylidene fluoride, e.g. pentafluoropropene,chlorotrifluoroethylene, and trifluoroethylene.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, VDF/HFP copolymers are preparedwhich have improved properties. Prior art methods for copolymerpreparation by emulsion polymerization wherein all of the HFP monomer iscombined with VDF monomer in an initial reaction mixture charge, orintroduced continuously in a fixed feed ratio with VDF, generally resultin the formation of uniformly random VDF/HFP copolymers. In contrast,VDF/HFP copolymers of the present invention, produced byhomopolymerizing VDF monomer, followed by further VDF polymerizationwith HFP, results in a polymer having discrete VDF homopolymer domains,and a discrete VDF/HFP copolymer domain. Such polymers possess improvedproperties over the uniformly random VDF/HFP copolymers. Specifically,they display greater melt flow, lower flexural modulus, improved impactresistance, and/or improved chemical resistance, but without sacrificeto the use temperature of the polymer. The aforementioned improvementsmay be realized in the production of resins according to the practice ofthe invention useful for various applications within the presentspectrum of commercial PVDF-based polymer usage. In particular, thepolymers of the invention provide low melt viscosity resins whichdisplay excellent flow characteristics under minimal sheer stress, whilenevertheless maintaining the integrity of favorable mechanicalproperties.

While conventional VDF/HFP copolymers display melt temperatures belowabout 150° C., generally 140°-145° C., the copolymers of the inventionare generally characterized by a melting point, as determined by DSC, inthe range of about 160° C. to about 170° C., more particularly fromabout 163° C. to about 168° C., which closely approaches the meltingpoint of VDF homopolymer generally (168°-170° C.). While certain highviscosity polymers prepared according to the present invention may becharacterized by a melting point somewhat below 160° C., it is notedthat the melting point nonetheless approximates the melting point of VDFhomopolymers formed using substantially the same initiator and operatingtemperature.

According to the process of the invention, a pressurized polymerizerreactor equipped with a stirrer and heat control means is charged withdeionized water, a conventional water-soluble surfactant, preferably awater-soluble fluorosurfactant, and optionally a hydrocarbon wax or oilto coat the metal surfaces of the reactor vessel to minimize adhesionpolymerization. The use of a horizontal polymerizer reactor is preferredsince it has less tendency to cause agitation-induced coagulation duringpolymerization.

Water-soluble fluorosurfactants which may be utilized include, forexample, those described in U.S. Pat. No. 2,559,752, e.g. acids of theformula X(CF₂)_(n)COOH wherein n is an integer from 6 to 20, X ishydrogen or fluorine, and the alkali metal, ammonium amine, andquaternary ammonium salts of the aforesaid acids; phosphoric acid estersof polyfluoroalkanols, of the formula X(CF₂)_(n)CH₂OPO-(OM)₂ where X ishydrogen or fluorine, n is an integer from 5 to 10, and M is hydrogen,alkali metal, ammonium, substituted ammonium (e.g., alkylamine of 1 to 4carbon atoms) or quaternary ammonium; sulfuric acid esters ofpolyfluoroalkanols, of the formula X(CF₂)_(n)CH₂OSO₃M where X and M areas above; the acids described in U.S. Pat. No. 3,232,970 of the formulaCF₂ClC(CF₃)F(CF₂)_(n)COOH where n is an integer of 3 to 9, and the metalsalts, ammonium salts, and acyl halides of said acid; the acids andsalts thereof described in U.S. Pat. No. 3,311,566 and mixtures thereofof the formula ZC_(n)F_(2n)COOM where Z is fluorine or chlorine, n is aninteger of 6 to 13, and M is hydrogen, alkali metal, ammonium, or loweralkyl substituted ammonium. The foregoing surfactants are meant to beillustrative of and not exclusive of fluoroalkyl surfactants, manyothers of which are known in the art and are operable as dispersionstabilizers in the emulsion polymerization of vinylidene fluoridepolymers. The preferred fluorosurfactants comprise ammonium salts ofperfluorocarboxylic acids having from 8 to 12 carbon atoms.

In the initial reactor charge , the concentration of thefluorosurfactant is from about 0.05 to about 0.5 wt. % (based on thetotal weight of the monomers utilized in the polymerization process,i.e., the combined weight of VDF and HFP added to the reaction medium),the preferred concentration being from about 0.1 to about 1.5 wt. %.

The concentration of the paraffin wax may vary over a wide range,according to the interior surface area of the reactor vessel. Generally,the wax concentration may vary from about 5 to about 20 mg per squareinch of inside surface area of the reactor vessel. Most preferably, theamount is 10 mg/in². The function of the wax is to prevent polymeradhesions to the reactor vessel. The wax is conventional. Any long chainsaturated hydrocarbon wax or oil may be used.

A chain transfer agent is optionally employed to regulate the molecularweight, and therefore the melt viscosity, of the polymer product. Theamount of chain transfer agent used, regardless of when it is addedduring the process, is a function of the desired melt viscosity andmolecular weight distribution of the reaction product. All or part ofthe optional chain transfer agent may be added to the initial charge.Generally, where a polymer having a low melt viscosity and narrowmolecular weight distribution is desired, such as in rotomoldingapplications, larger amounts of chain transfer agent are utilized.Representative chain transfer agents include various alcohols andketones, such as acetone, t-butanol and isopropyl alcohol (IPA). See,for example, U.S. Pat. Nos. 3,857,827 and 4,360,652, disclosingpolymerization of VDF using acetone and IPA, respectively, as chaintransfer agents.

The polymerization may be initiated utilizing any of the variousinitiators, which typically comprise organic peroxy compounds, usefulfor the polymerization of vinylidene fluoride. Two classes of peroxycompounds particularly useful as polymerization initiators comprise theorganic peroxides, e.g., di(tert-butyl) peroxide (“DTBP”) and thedialkyl peroxydicarbonates, most particularly the dialkylperoxydicarbonates wherein the alkyl groups comprise straight orbranched carbon chains containing from 1 to 8 carbon atoms. The lattercategory includes, for example, di(n-propyl) peroxydicarbonate,di(sec-butyl) peroxydicarbonate, di(ethylhexyl) peroxydicarbonate anddiisopropyl peroxydicarbonate (“IPP”).

Where IPP is employed as the polymerization initiator, the use ofacetone or IPA as the chain transfer agent may lead to development ofvoids of 15% or greater in the polymer resin if exposed to temperaturesgreater than 550° F. Thus, where IPP is utilized as the initiator, thepreferred chain transfer agent is TCFM. As set forth in U.S. Pat. No.4,569,978 at column 3, lines 41-54, TCFM provides advantages withrespect to lowering initiator consumption, reducing polymer voids, anddecreasing the discoloration of the resulting polymer. TCFM may beutilized in varying concentrations to produce products with a wide rangeof molecular weights without varying the initiator concentration.Generally, the amount of TCFM utilized may vary from about 0.5 to about5.0 wt. %, more preferably from about 1.5 to about 4.0 wt. %, based uponthe total amount of monomers added to the reaction medium during theprocess. TCFM is particularly preferred as the chain transfer agentwhere preparation of polymers having a melt viscosity of less than about15 kilopoise is desired.

After the polymerizer reactor has been charged with water, surfactant,wax and optionally, chain transfer agent, and before the introduction ofmonomer, the reactor is subjected to a series of evacuations andnitrogen purges to insure an oxygen-free environment for thepolymerization. Optionally, before introduction of the monomer, thereactor may be deaerated with a “boil-out” procedure during which theaqueous charge in the reactor is heated to 100° C. while agitating. Oncethe aqueous charge is boiling, the agitation is stopped and the steamand air are vented off. After about 1 to 10 minutes, typically 2 to 3minutes, the reactor is sealed and agitation continued. Both methods areessentially equivalent and important for the successful start of thepolymerization reaction.

The system is sealed and brought to the desired reaction temperature,i.e. from about 65° C. to about 150° C., preferably from about 65° C. toabout 105° C., most preferably from about 75° C. to about 90C. Thedesired reaction temperature depends, in part, on the nature of thepolymerization initiator employed. For IPP, the preferred initiator, thereaction temperature is preferably from about 65° C. to about 105° C.For DTBP, the reaction temperature may be permitted to reach about 150°,preferably no higher than about 135° C. Sufficient VDF is charged toobtain the desired operating pressure. The operating pressure may varywithin broad limits. Preferably the pressure is from, for example, about450 to about 850 psig, preferably from about 550 to about 650 psig. TheVDF requirement for the initial VDF charge varies directly with theoperating pressure and inversely with the operating temperature.

In the next step, to begin homopolymerization of VDF, an initial chargeof polymerization initiator is added. The preferred polymerizationinitiator is IPP. DTBP, among other initiators, is also useful. In thepolymerization induction period, IPP is added in an amount sufficient toachieve a concentration of from about 0.25 to about 2.0 grams IPP perkilogram of VDF monomer present in the initial reactor charge. Mostpreferably, the initial concentration of IPP is from about 0.5 to about1.5 grams per kilogram of the VDF monomer in the initial reactor charge.

When the initial charge does not contain chain transfer agent, theaddition of VDF monomer should be followed by the addition of chaintransfer agent until the ratio of chain transfer agent to monomer to bemaintained throughout the reaction is reached. The chain transfer agentmay be added in its entirety to the initial charge, or added in itsentirety after the initial charge, or may be added to both the initialcharge and after the initial charge.

Following the polymerization induction period, VDF monomer,polymerization initiator, and optional chain transfer agent, are fedcontinuously or incrementally in a constant ratio and at a rate whichprovides an essentially constant pressure within the polymerizer. Theinitiator concentration generally controls the rate of polymerization.Therefore, as the polymerization of VDF proceeds, the amount ofinitiator added is dependent on the rate of reaction which is desired.Economically it is desirable to run the polymerization reaction as fastas possible, with the limiting factor being the capacity of thepolymerizer cooling jacket to remove the heat of polymerization from thereaction vessel. The feed rate of initiator is advantageously adjustedto maintain the desired VDF polymerization rate, e.g. a rate of fromabout 1000 to about 1100 pounds per hour, commercial scale.

The homopolymerization of VDF proceeds until about 50-90%, preferablyabout 65-85%, most preferably about 70-80%, by weight of the VDFutilized in the emulsion polymerization process has been added to thereaction mixture. At this point, HFP monomer is added to thepolymerization mixture in an amount of from about 1% to about 20% byweight, based upon the combined weight of the HFP and the total weightVDF monomer added to the reaction medium in the polymerization process.Below 1% HFP, the resulting polymer essentially has the properties ofVDF homopolymer. Above 15% HFP, the polymer begins to take on theproperties of a fluoroelastomer, which properties become pronouncedabove about 20% HFP. Since fluoroelastomers are generally not meltprocessable, it is preferred to limit the HFP content of the presentVDF/HFP copolymers to not more than about 20%, most preferably not morethan about 15%. Since HFP reacts more slowly than VDF, it is preferredthat substantially the entire amount of HFP is added to the reactionmedium as quickly as possible at the appropriate time to maximize theopportunity for the HFP monomer to react. The VDF feed is preferablycontinued during the HFP feed period, preferably at the same orsubstantially the same rate as before the addition of HFP.

If the HFP is added to the reaction medium before at least about 50% ofthe VDF has been added, the melting point of the resulting VDF/HFPcopolymer will be depressed. If more than about 90% of the VDF utilizedin the process is added to the reaction medium before the HFP is added,sufficient HFP might not be incorporated into the polymer to obtainsignificant improvement in the physical properties of the polymer overthe properties of PVDF homopolymer.

The rate of HFP addition which should be employed is a function of thetiming of the HFP introduction. Where HFP is added to the reactionmedium late in the polymerization process, e.g., following addition of90% of the total weight of VDF utilized in the process, fairly rapidaddition of the HFP is required to ensure that it has adequateopportunity to react. Where the HFP is introduced early in thepolymerization process, e.g., after only 50% of the total VDF weight hasbeen added to the reaction medium, a somewhat slower rate of HFP may beutilized. Regardless of the timing of the addition, HFP should beintroduced substantially completely over a small interval of time incomparison with the interval of VDF addition, and not progressively withthe VDF feed.

When all the VDF has been introduced, all feeds are discontinued and thebatch is allowed to react-out for 30 minutes. Generally, when thepressure has dropped to 150-200 psig, no further reaction takes place.At this time, the agitation is also stopped and the residual monomer(s)are vented through a valve mounted over the vapor space of the reactorto a gas holder for subsequent recycling.

Upon completion of the polymerization, the product is recovered as alatex which may be optionally concentrated by creaming or flashevaporation, or the latex may be coagulated and the polymer recovered asa powder. The latex consists of a stable mixture of the reactioncomponents, i.e., water, surfactant, initiator or initiatordecomposition products, chain transfer agent, etc., along with polymerresin particles whose inherent size ranges from between about 0.2 toabout 0.3 microns. The polymer resin particles may be isolated by avariety of methods known to those skilled in the art, such as, forexample, acid coagulation, freeze coagulation, addition of saltingagents, or mechanical shear resulting in the formation of particleflocks. The resin is then recovered from the coagulated material bywashing and drying.

The resulting copolymer has an HFP content, of from about 1% to about20%, preferably from about 1% to about 15%, most preferably from about5% to about 15% by weight.

The practice of the invention is illustrated in the followingnon-limiting examples. All examples were carried out at 80-gallon scale,except Example 4, which was carried out at 2-gallon scale. The Exampletitles include the relative proportions of VDF and HFP monomers fed tothe reaction medium in the polymerization process. The HFP content ofthe resulting polymers is generally about 74% to about 83% of the HFPfeed, as a portion of the HFP fed to the reactor typically does notenter into the polymerization reaction. Thus, in Example 1 for instance,while the monomer feed comprised 10 wt. % HFP, the HFP content of theresulting polymer was only about 8 wt. %, reflecting a 20%non-utilization of HFP monomer.

COMPARATIVE EXAMPLE 1 88/12 VDF/HFP Low Viscosity Polymer (2.3kilopoise)

The following comparative example is based upon Example 15 of U.S. Pat.No. 4,569,978 except that the melt viscosity was reduced from 22.1kilopoise to 2.3 kilopoise by decreasing the amount of chain transferagent (TCFM) to 6 lbs., which was all added in the initial charge,rather than continuously.

Into an 80-gallon stainless steel autoclave, 454 pounds (55 gallons) ofdeionized water, 100 grams of ammonium perfluorodecanoate (surfactant),and 12 grams of a paraffin wax were charged. The reactor was closed,evacuated and heated to 90° C. with agitation (23 rpm, corresponding to113 surface feet per minute). The following were pumped into the reactorto obtain the desired operating pressure of 550 psig: 18.5 pounds VDFmonomer; 2.5 pounds HFP monomer; 6 pounds TCFM. When operatingconditions stabilized, the polymerization was begun by introducingapproximately 1 pound of IPP initiator. The initiator was added as anemulsion consisting of 1 wt. % IPP in deionized water containing 0.15wt. % ammonium perfluorodecanoate. The rate of addition of the IPPemulsion was adjusted to obtain and maintain a polymerization rate of 60pounds/hour of combined VDF and HFP monomers, which were fed throughoutthe reaction in a fixed 88/12 weight percent ratio. In approximately 3hours, 176 pounds of VDF and 24 of pounds HFP had been added to thereaction, and all feeds were stopped. The batch was allowed to react-outat a constant temperature of 90° C. to consume residual monomers atdecreasing pressure. After about 30 minutes, the agitation was stoppedand the reactor was vented, and the latex recovered. Polymer resin wasisolated by coagulating the latex, washing the latex with deionizedwater, and drying. The resin comprised a random copolymer having a meltviscosity of 2.3 kilopoise measured at 232° C. and 100 sec⁻¹ (ASTMD3835), and a DSC melting point of 140-145° C.

Example 1 90/10 VDF/HFP Low Viscosity Polymer (1.4 kilopoise )

Into an 80-gallon stainless steel reactor was charged, as in the mannerof Comparative Example 1, 454 pounds of deionized water, 100 gramsammonium perfluorodecanoate and 12 grams of paraffin wax. Followingevacuation, agitation was begun and the reactor was heated to 90° C., asin Comparative Example 1. During heat up of the ingredients, VDF monomer(approximately 20 pounds) and TCFM (6.5 pounds) were added. Uponstabilization of the temperature and operating pressure, 1 pound of IPPin the form of an emulsion consisting of 1 wt. % IPP in deionized watercontaining 0.15 wt. % ammonium perfluorodecanoate was added to begin thepolymerization. The rate of further addition of the IPP emulsion wasadjusted to obtain and maintain a VDF polymerization rate of 60 poundsper hour. The VDF homopolymerization reaction was continued untilapproximately 135 pounds (representing 75 wt. % of the total VDF monomerutilized in the herein example) was introduced into the reaction mass.Thereafter, 20 pounds of HFP (comprising 10 wt. % of the total weight ofcombined VDF and HFP monomers utilized in the herein example) was pumpedinto the reactor at a rate of approximately 100 pounds per hour, whilethe VDF feed was continued. The sudden influx of the relatively slowreacting HFP monomer temporarily suppressed the reaction rate. Theinitiator addition rate was increased to restore the polymerization rateback to 60 pounds per hour. The reaction continued until a total of 180pounds of VDF had been added to the reaction mass. The react-out cycleand resin recovery process was repeated as in Comparative Example 1.

The resulting resin displayed a melt viscosity of 1.4 kilopoise,measured at 232° C. and 100 sec⁻¹ (ASTM D3835), and a DSC melting pointof 163-168° C.

Example 2 90/10 VDF/HFP Hiqh Viscosity Copolymer (15.4 kilopoise)

The procedure of Example 1 was repeated except that the amount of TCFMwas reduced from 6.5 pounds to 3.2 pounds in order to produce acopolymer of higher melt viscosity, suitable for extrusion applications.The amount of IPP in the initial charge was reduced to 0.05 wt. % basedupon the weight of VDF monomer present in the initial charge, and thenIPP was fed at a rate sufficient to maintain a polymerization rate of 60pounds per hour. The resulting copolymer displayed a melt viscosity of15.4 kilopoise, measured at 232° C. and 100 sec⁻¹ (ASTM D3835), and aDSC melting point of 163°-168° C. The material, suitable for extrusionapplications, was characterized by a break elongation of about 350-450%,as measured by ASTM D882. In contrast, VDF homopolymer is characterizedby a break elongation of only about 50-250%.

Example 3 95/5 VDF/HFP Medium Viscosity Copolymer (8-10 kilopoise)

The procedure of Example 1 was repeated except that the amount of TCFMadded to the reaction medium was 4.4 pounds (2.2 wt. %, based upon theweight of the combined monomers) to yield a product having a meltviscosity intermediate between the melt viscosities of the products ofExamples 1 and 2, suitable for injection molding. Furthermore, the levelof HFP added to the reaction medium was reduced to 5 wt. % of thecombined monomers. The polymer product displayed a melt viscosity of8-10 kilopoise at 232° C. and 100 sec⁻¹ (ASTM D3835), and a DSC meltingpoint of 163-168° C.

Example 4 95/5 VDF/HFP High Viscosity Copolymer (29-33 kilopoise)

Into a 2-gallon reactor were charged 5145 g (11.33 lbs.) of deionizedwater, 2.3 g ammonium perfluorodecanoate, and 4 g of paraffin wax.Following evacuation, agitation was begun and the reactor was heated to125° C. During heat-up of the ingredients, 9 ozs. of VDF monomer wereadded. Upon stabilization of the temperature and operating pressure,approximately 4 g DTBP initiator were added to begin the polymerization.The foregoing amounts of VDF and DTBP represent 1 weight fraction and 2weight fractions, respectively, of the total amounts of these reactantsutilized in this Example. Subsequent additions of the DTBP initiatorwere maintained at the same ratio until all the DTBP had beenintroduced. At the point at which all of the DTBP was added, 50% of theVDF had been introduced. The VDF addition was continued untilapproximately 60 ozs. (representing 75% of the total mount of VDFutilizing in this Example) were introduced to the reactor. At thatpoint, 0.25 lb. of HFP (comprising 5 wt. % of the total weight ofcombined VDF and HFP monomers used herein) was pumped into the reactorat a rate of 2.5 pounds per hour, while the VDF feed was continued. Thereact-out cycle and resin recovery process was repeated as inComparative Example 1. The resulting polymer was characterized by a meltviscosity of 29-33 kilopoise at 232° C. and 100 sec⁻¹ (ASTM D3835), anda DSC melting point of 152-156° C.

It should be noted that the polymerization initiator utilized in Example4, DTBP, is relatively slow-reacting compared to the IPP initiator ofExamples 1-3. The half-life of DTBP is 10 hours at 125° C. In contrast,the half-life of IPP at 75° C. is only 15 minutes. Accordingly, it isnecessary to add DTBP in larger quantities (5.5-7.5 g/kg of monomer),and at a faster rate in comparison to IPP, in order to provide asufficient number of initiator sites to achieve an overallpolymerization rate of 1.5 lbs. per hour, which corresponds to 1000-1500lbs. per hour at commercial scale.

The physical properties of representative batches of the ComparativeExample 1 and Examples 1-3 materials are set forth in Table 1, below.Two batches each of the Comparative Example 1 and Example 1 materialswere tested. The values of some measured parameters, e.g. meltviscosity, typically vary slightly from batch to batch with slightvariations in the HFP content, amount of chain transfer agent, and thelike.

The properties of the Example 2 VDF/HFP copolymer are compared to theproperties of the commercially available VDF homopolymer having asimilar melt viscosity (KYNAR® 730, Atochem North America, Inc.).Likewise, the properties of the Example 3 VDF/HFP copolymer are comparedto the properties of a commercially available VDF homopolymer having asimilar melt viscosity (KYNAR® 720, Atochem North America, Inc.). Thefollowing ASTM test procedures were employed:

Specific Gravity ASTM D792 Tensile Strength ASTM D882 (yield) TensileStrength ASTM D882 (break) % Elongation ASTM D882 (break) FlexuralModulus ASTM D790 T_(m) (DSC) ASTM D3418 T_(c) (DS) ASTM D3418 MeltViscosity ASTM D3835

TABLE 1 Compar. Ex. 1 Example 1 Batch A Batch B Batch A Batch B Example2 PVDF² Example 3 PVDF³ Specific gravity (1.77-1.79) (1.77-1.79)1.77-1.79 1.77-1.79 1.77-1.79 1.77-1.79 Tensile Strength 3120 3565 44534345 3957 5000-7000 5710 5000-7000 (yield) psi Tensile Strength 42553100 2229 2141 5390 4000-6200 3685 4200-7000 (break) psi % Elongation600 392 218 320 472  50-250 393  50-250 (break) Flexural Modulus, 89 105141 105 107 180-300 158 200-325 Kpsi Dart impact Ductile ND Ductile ND¹Ductile Ductile Ductile Slightly strength brittle T_(m) ° C. 144.9 144.5166.4 166.5 164.0 165-170 169.4 165-170 T_(c) ° C. 103.0 109.7 129.2127.4 135.9 130-135 137.0 130-135 HFP content, wt. % 10.8 9.80 7.40 8.307.80 0 4.10 Melt viscosity, 2.90 2.60 3.70 4.0 15.0 13.5-16.5 8.10 7.5-10.5 kilopoise ND¹ = not done PVDF² = KYNAR ® 730 PVDF³ = KYNAR ®720

The melting temperature (T_(m)) was determined by differential scanningcalorimetry during the second temperature scanning cycle. The crystalphase transition temperature (T_(c)) was determined by differentialscanning calorimetry during the first temperature scanning cycle. TheDart impact resistance test was conducted by dropping a 35 pound weighthaving a ½ inch diameter tup from a height of 18 inches onto a 4×4 inchplaque ({fraction (1/16)} inch thickness) of the polymer materialclamped in a circular holding device leaving a 3¾ inch area open forimpact with the tup.

The properties of the Example 4 VDF/HFP high viscosity copolymer werecompared to the properties of a commercially available VDF homopolymerhaving substantially the same melt viscosity range (KYNAR® 460, AtochemNorth America, Inc.). Table 2 sets forth the significantly beneficialdifferences in properties:

TABLE 2 Example 4 PVDF¹ Notched Impact 9.4 2 to 3 Strength, ft-lbs perinch of notch² Chemical Stress Crack >256⁴ 12 to 46 Resistance, days for60% failure³ DSC Melt Point, ° C. 152 to 156 155 to 160 ¹KYNAR ® 460(Atochem North America, Inc. ²ASTM D256. ³Specimens under 20% strain in10 wt. % NaOH at 90° C. ⁴Only 1 failure - 4 of 5 still intact.

The products of the present invention yield an opaque polymersignificantly whiter in color than PVDF resins presently available.Moreover, the products of the invention display significantly greaterresistance to selected chemical agents than corresponding uniformlyrandom VDF/HFP copolymers having a similar HFP content.

Rotomolding applications require a resin which exhibits excellent meltflow capability at essentially zero stress. While the desired melt flowmay be achieved with very low molecular weight PVDF polymers, themechanical properties of rotomolded articles prepared from suchhomopolymers are unacceptable. Elongation and impact resistance, inparticular are poor. The present invention provides for rotomoldinggrade resins, as illustrated by the resin of Example 1, which haveexcellent mechanical properties without sacrificing maximum usetemperature. The uniformly random VDF/HFP 88/22 copolymer of ComparativeExample 1, while having excellent flow characteristics (meltviscosity=2.3 kilopoise) is characterized by a DSC melting point of only140-145° C., making it unsuitable for high temperature applications. The90/10 VDF/HFP copolymer of Example 1 on the other hand, while havingroughly a similar HFP content, possess excellent flow characteristics(melt viscosity=1.4 kilopoise), and retains a maximum use temperature of163° C.-168° C., which approaches the use temperature of PVDFhomopolymer. The Example 1 material, which combines extremely lowviscosity with excellent impact strength and a high use temperature, ischaracteristic of rotomolding VDF/HFP copolymer resins made possible bythe practice of the present invention.

Particularly useful for rotomolding applications are VDF/HFP copolymersaccording to the invention having melt viscosities in the range of fromabout 1 to about 4 kilopoise, measured at 232° C. and 100 sec⁻¹ (ASTMD3835). Such copolymers are also useful for forming powder coating resincompositions having. improved flexibility and crack resistance accordingto the commonly assigned U.S. patent application Ser. No. 521,792entitled “Powder Coatings of Vinylidene Fluoride/HexafluoropropyleneCopolymers” of Michael D. Poleck, filed on May 10, 1990, now U.S. Pat.No. 5,177,150. The entire disclosure of the aforementioned commonlyassigned patent application is incorporated herein by reference.

In some applications, parts extruded from conventional PVDF resins maybecome highly stressed due to post-forming operations, such as flangeforming on pipe lining ends. In such operations, the part may bestretched to near the break elongation of the extruded resin, which maycause flange failure after extended service. Exposure to certainchemical environments may accelerate the tendency of the article tostress crack. The invention provides for the production of extrusiongrade VDF/HFP copolymer resins suitable for forming pipe, pipe lining,and the like. Such extrusion grade resins, as illustrated by Example 2,are characterized by an increased break elongation in comparison to PVDFhomopolymer. The increase in break elongation simplifies fieldflange-forming operations, and provides flanges of greater strength andflexibility than heretofore possible utilizing PVDF resins.

Injection molding applications generally require a resin having a meltviscosity lower than extrusion grade, but higher than rotomolding grade.Fittings prepared by injection molding from conventional PVDF resins canexhibit significant discoloration which is apparent and objectionablewhen the fittings are fusion welded to lengths of pipe which are muchwhiter. Processing conditions may be modified to produce fittings whichmatch the color of the pipes, but such parts are inherently brittle. Theinvention enables the injection molding of VDF/HFP copolymer resins intochemical process industry fittings, which are characterized by physicalproperties intermediate between rotomolding and extrusion grades. Thepolymer of Example 3 is illustrative of such an injection molding gradepolymer prepared according to the present invention. Fittings formedfrom injection molding grade resins of the invention are ductile, ratherthan brittle, and do not suffer from discoloration.

The polymer of Example 4 represents a high viscosity polymer (29-33kilopoise) prepared without employing a chain transfer agent separatefrom the polymerization initiator, DTBP. The Example 4 polymer issuitable for extrusion, compression molding and injection molding. Itmay be appreciated from a consideration of Table 2 that while themelting point range of the high viscosity Example 4 polymer is somewhatlower than 160° C., the range (152-156° C.) nevertheless approximatesthe melting point range 155-16° C.) of a VDF homopolymer formed usingsubstantially the same initiator and operating temperature.

The particular resin grade desired—rotomolding, injection, orextrusion—may be obtained primarily by manipulating the amount andnature of the chain transfer agent and the amount of HFP added to thereaction medium in the polymerization process. Generally, theutilization of greater amounts of chain transfer agent, particularlyTCFM, results in polymers having low molecular weights, and thereforelow melt viscosities. Thus, the Example 4 polymer, which was preparedwithout chain transfer agent, displayed the relatively high meltviscosity of 29-33 kilopoise. The impact resistance and increased breakelongation properties are primarily dependent upon the HFP content ofthe polymer.

The process of the present invention results in VDF/HFP copolymershaving a chemical composition distinct from that of random VDF/HFPcopolymers, as illustrated by the following comparison.

A 88/12 VDF/HFP random copolymer prepared by incremental addition of HFPto an emulsion polymerization reaction medium containing VDF, resultedin an essentially uniform copolymer with HFP units regularly distributedwithin the polymer chains. The Example 1 copolymer, prepared by additionof HFP after 75% of the VDF had been fed to the polymerization reactionmedium, was characterized by an irregular HFP distribution, asdetermined by nuclear magnetic resonance analysis of selectedsolution-fractionation samples.

Solution-fractionation was accomplished by exposing a film of eachpolymer to refluxing mixtures of acetone/methanol of graduallyincreasing acetone concentration. Solid polymer fractions were isolatedby distillation of the solvent and drying of the residues. Data areshown in Table 3.

TABLE 3 VDF/HFP COPOLYMER FRACTIONATION Vol. % Wt. % Cum. Wt. % FractionAcetone Soluble Soluble Random Copolymer A 30 0.92 0.92 B 40 7.35 8.25 C50 38.25 46.5 D 60 58.25 100 Example 1 Copolymer A′ 30 3.82 3.82 B′ 402.53 6.35 C′ 50 4.54 10.89 D′ 60 5.97 16.86 E′ 70 5.85 22.71 F′ 80 5.2127.92 G′ 90 65.33 93.25 H′ 100 1.18 100

The approximate middle fraction (C) and major fraction (D) of the randomcopolymer, and the approximate middle fraction (D′) and major fraction(G′) of the Example 1 copolymer were subjected to fluorine-19 nuclearmagnetic resonance spectral determination. Data are shown in Table 4.

TABLE 4 Solvent Sequence Fraction Vol. % % HFP(2) Distribution Sample %Sol.(1) Acetone Mol. % Wt. % HFP-VDF-HFP(3) Random Copolymer C 38.3 504.5 9.9 0.4 D 58.3 60 4.2 9.3 0.3 Example 1 D′  6.0 60 9.2 19.2  1.8 G′65.3 90 2.1 4.8 0.4 (1) Fraction of the polymer which dissolved in amixture of acetone/methanol with the listed acetone content. (2)Fluorine-19 NMR Spectral Determination (3) HFP-VDF-HFP sequence =

It may be observed that the HFP unit concentration of the approximatemiddle acetone/methanol fraction (D′) of the Example 1 copolymer is muchhigher than the HFP unit concentration of the major Example 1 fraction,(G′) or the random copolymer fractions (C,D). Similarly, the-HFP-VDF-HFP-sequence distribution in the Example 1 middle fraction (D′)is found at a much higher frequency than in the major Example 1fraction, (G′) or in the random copolymer fractions (C,D). It isbelieved that the irregular distribution of HFD/HFP comonomer, and theirregular occurrence of the -HFP-VDF-HFPsequence, accounts for theunique properties of the copolymers of the invention.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

What is claimed is:
 1. A vinylidene fluoride-hexafluoropropylenecopolymer comprising one or more discrete vinylidenefluoride-hexafluoropropylene copolymer domains and one or more discretevinylidene fluoride homopolymer domains including a major vinylidenefluoride homopolymer domain which contains at least about 50% of thevinylidene fluoride content of the copolymer, said copolymer having ahexafluoropropylene content of from about 1% to about 20% by weight ofthe copolymer.
 2. A copolymer according to claim 1 wherein said majorvinylidene fluoride homopolymer domain contains at least about 65% ofthe vinylidene fluoride content of the copolymer.
 3. A copolymeraccording to claim 2 wherein said major vinylidene fluoride homopolymerdomain contains at least about 70% of the vinylidene fluoride content ofthe copolymer.
 4. A copolymer according to claim 1 having a meltingpoint in the range of from about 160° C. to about 170° C.
 5. A copolymeraccording to claim 4 having a melting point in the range of from about163° C. to about 170° C.
 6. A copolymer according to claim 1 having ahexafluoropropylene content of from about 1% to about 15% by weight ofthe copolymer.
 7. A copolymer according to claim 6 having ahexafluoropropylene content of from about 5% to about 15% by weight ofthe copolymer.
 8. A copolymer according to claim 2 having ahexafluoropropylene content of from about 5% to about 15% by weight ofthe copolymer.
 9. A copolymer according to claim 3 having ahexafluoropropylene content of from about 5% to about 15% by weight ofthe copolymer.
 10. A copolymer according to claim 1 having a meltviscosity in the range of from about 1 to about 4 kilopoise, measured at232° C. and 100 sec⁻¹.